1. Field of the Invention
This invention relates to a process for the manufacture of silicic acid esters by esterification of chlorosilanes and/or chlorosiloxanes with primary alcohols. This invention also relates to oligomerization effected simultaneously with esterification or immediately thereafter, by addition of water, to form compounds of the formula ##STR1## wherein R represents an alkyl radical of 1 to 12 carbon atoms, which alkyl radical can contain a hetero atom in the chain, such as oxygen or sulfur,
n represents a whole number or, in the case of oligomeric mixtures, a fraction as well, between 1 and 10, preferably between 1 and 6.
2. Discussion of the Prior Art
It is known to form orthosilicic acid esters or their oligomeric condensation products by reacting chlorosilanes with alcohols. The known processes suffer from several drawbacks owing to the formation of a chain of by-products. These by-products appear regularly in more or less concentrated amounts. The essential side reaction prevailing is the reaction whereby developing hydrogen chloride reacts with alcohol reactant to form water, alkyl chloride and dialkyl ethers. The last two compounds pollute the hydrogen chloride being generated and make it unsuitable for the production of highly pure trichlorosilane which is useful in the field of transistors. The organic compounds contain pollutants such as methyldichlorosilane under the conditions of trichlorosilane synthesis. These pollutants cannot be readily removed from the trichlorosilane and therefore provide inferior transistor silicon when the process is conducted to form highly purified silicon.
Additionally, the water formed interferes by way of hydrolysis with the esterification reaction of the chlorosilane and forms a residue consisting of oligomeric and polymeric siloxanes which are useless for reasons of their undefined structure. These by-products represent considerable loss of material and reduce yield. Moreover, the maneuverability of the esterification reaction becomes particularly difficult at the point where oligomeric silicon esters of desired structure or of certain polymers should be produced because the water which develops in the undesired side reaction blocks the occurrence of these compounds. In doing so, undesirable polycondensates develop which cannot be separated from the desirable polycondensates and which therefore lower the quality of the end product.
Another drawback, customary with reactions of hydrogen chloride in the conventional process, occurs in the production of such silicic acid esters whose alkyl radicals contain hetero atoms such as oxygen in ether compounds. The developing hydrogen chloride acts ether-cleavage and leads to an undesirable content of organically bound chlorine within the end product. The by-products contain .beta.-chloroethoxy groups which possess in part highly toxic properties, as is known.
As a consequence, it became desirable to provide a process for the complete esterification of tetrachlorosilane and its oligomeric homologs, which process could be conducted in such a way as to obtain the desired ester in the highest possible yield. More especially, it became desirable to provide a process which could be conducted with realization of as few pollutants as possible. In addition, it became desirable to provide a process wherein the hydrogen chloride resulting from the process would be of sufficient purity that it could be reacted with ferrosilicon or silicon to form highly pure halosilanes.